It uses __get_cpuid_count, provided by cpuid.h.
The xgetbv is implemented after its counterpart in cpu_x86.s.

Change-Id: I97624d7da67ab4ec3c9a53b0bfc4dfcdf7d12c87
Reviewed-on: https://go-review.googlesource.com/116155Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>